In multiple beam lithography, such as charged particle beam lithography, small structures may be formed, i.e. written, with high accuracy and reliability. In charged particle lithography charged particles are directed onto a target surface, typically a wafer surface, to form patterns which may form the basis for integrated circuits and components thereof. In multiple beam lithography the pattern formed on the surface is determined by the position where each individual beam interacts with the resist on the surface.
Lithographic processing generally involves multiple exposures of layers, so that features formed in subsequent layers may be connected to create an integrated circuit. Therefore, not only each pattern itself has to meet the required accuracy, but it is also a requirement that a pattern exposed in a later exposure session is aligned with the one or more patterns created in earlier exposure sessions in a sufficiently accurate manner.
While in optical lithography the pattern is formed by illumination of a target surface through a mask, the accuracy and quality of the resulting pattern influenced by the accuracy of the mask, in multiple beam lithography the pattern is determined by the position where each individual beam interacts with the surface. In multiple beam lithography it is therefore of importance that each individual beam is correctly positioned, such that it impinges at the intended position on the surface in order that the intended pattern is formed. This is often referred to as pattern placement accuracy.
Various technologies for measuring beam properties and pattern placement properties have been developed.
US 2012/0268724 A1 describes methods and systems for aligning the target with the optical column of a multiple beam lithography apparatus. Further, determination of a spatial distribution of beam properties are described, using a beam measurement sensor positioned on the chuck.
U.S. Pat. No. 7,868,300 B2 and US 2012/0293810 A1 describe methods and systems for measuring beam properties using a sensor having a surface comprising beam blocking and non-blocking regions, also known as knife edge measurements. The sensor surface may be positioned at a position corresponding to the position of the target surface during lithography.
Although methods and systems described in the documents cited above enable measurement of beam properties, such as spatial distribution of beams, these do not enable verification of the pattern placement accuracy.
A known method for verifying the result of a lithography process is offered by scanning electron microscopy (SEM), such as critical dimension SEM (CD-SEM). In CD-SEM the CD-uniformity can be verified by measuring dimensions of features formed on the surface. While SEM offers high enough resolution for studying dimensions of interest in semiconductor technology, the field of view is limited to surface areas with dimensions of a few μm2. Therefore, it is not possible to observe a pattern having contributions from each individual beam used during the multiple beam lithography. In order to verify pattern placement accuracy for all beams, the measurement procedure has to be repeated for several measurement areas. The CD-SEM method is therefore time consuming.